A solid state imager includes a focal plane array of pixel cells, each one of the cells including a photoconversion device, for example, a photogate, photoconductor, or a photodiode for accumulating photo-generated charge. Several different types of imagers are known, for example, CCD, CMOS and others. Almost all solid state imagers include a microlens array having individual lenses which respectively focus light on the photoconversion device of the pixel cells.
The microlenses help increase optical efficiency and reduce cross talk among pixel cells. As the resolution of the pixel cell array increases, there is a corresponding reduction of the size of the pixel cells arranged in a specific pixel cell array. Likewise, the size of the individual lenses in the microlens array also decreases.
It is important that a microlens focal point coincide with the part of an underlying photoconversion device which provide the greatest conversion efficiency of incoming photon to electrical charges. Accordingly, microlenses are arranged to have the focal point coincide with the center of a pixel cell. However, in some instances this may not coincide with the location of the photoconversion device or the portion of the photoconversion device having the greatest charge conversion efficiency.